Although enrofloxacin (ENR) is an important pharmaceutical in aquaculture for its excellent efficacy, it is mostly utilized through an off‐label prescription system. Thus, the optimal clinical use regimen is not well‐established. A series of studies was performed to obtain pharmacological and safety data needed for the use of ENR, a synthetic fluoroquinolone, in treating bacterial diseases occurring in black rockfish, Sebastes schlegelii. Minimal inhibitory concentrations showed that ENR exerts a potent in vitro efficacy inhibiting the growth of several pathogenic bacteria at concentrations less than 1 mg/L. ENR with oral doses of 5 mg/kg and up for 1 week significantly prevented mortality of black rockfish infected with Streptococcus iniae and Aeromonas salmonicida, demonstrating in vivo efficacy. Complete protection was obtained against these two bacteria by 10 mg/kg ENR in this in vivo infection trial. Black rockfish did not exhibit any severe abnormal signs by ENR up to 20 mg/kg when assessed by basic haematoclinical and histopathological parameters. Efficacy and toxicity study results collectively suggest that oral dose of 10 mg/kg ENR is optimal to treat bacterial infections. Through muscle residue analyses, the required withdrawal period for safe fish consumption was found to be approximately 90 days by measuring clearance of ENR and its metabolite ciprofloxacin (CIP). This study provides necessary information for the use of ENR to treat two frequently‐occurring bacterial infections, Streptococcus iniae and Aeromonas salmonicida in black rockfish. Obtained data can be the basis for an official guideline for aquacultural ENR use.

Background: Amoxicillin (AMOX) is widely used in aquaculture for bacterial infections due to its efficacy and safety. Despite official approval for select species, off-label use is common. This study evaluated the antibacterial efficacy and residue depletion of AMOX in five aquaculture species: olive flounder (Paralichthys olivaceus), rainbow trout (Oncorhynchus mykiss), Japanese eel (Anguilla japonica), black rockfish (Sebastes schlegelii), and Israeli carp (Cyprinus carpio). Methods: Fish were administered AMOX orally at 40 mg/kg and 80 mg/kg for seven days. Antibacterial efficacy was assessed by bacterial load reduction and survival rates following artificial infection. Residue depletion was analyzed using HPLC–MS/MS to determine the time required for AMOX levels to fall below the maximum residue limit (MRL, 0.05 mg/kg). Results: AMOX, at 40 mg/kg, significantly reduced bacterial loads in olive flounder, rainbow trout, and Japanese eel (p < 0.05), while Israeli carp exhibited a limited response (p = 0.54). Black rockfish showed moderate efficacy (RPS 72.7%) but increased mortality at 80 mg/kg. Residue levels fell below the MRL within 10 days for all species except Israeli carp (~30 days). Conclusions: These findings highlight species-specific differences in AMOX efficacy and residue depletion rates, emphasizing the necessity of tailored dosing regimens and withdrawal periods to ensure optimal therapeutic outcomes and food safety compliance in aquaculture. Further pharmacokinetic studies are needed to refine dosing strategies, particularly for species with extended residue retention and potential dose-dependent adverse effects.

This study evaluated the therapeutic efficacy of amoxicillin (AMX) or lincomycin (LCM) in starry flounders ( Platichthys stellatus ) infected with Streptococcus parauberis . Seventeen S. parauberis strains isolated from various regions of Korea were tested to determine the minimum inhibitory concentrations (MICs) of AMX and LCM. A representative strain (HFTC0091) was used for the experiments. Following bacterial infection, starry flounders were treated with either oral AMX (40 or 80 mg/kg/d for 7 days) or a single intramuscular (IM) injection of LCM (10 or 20 mg/kg). High‐dose treatments with both antibiotics resulted in significantly improved survival rates (96%–100% for AMX and 76%–96% for LCM) compared to nearly 100% mortality in the untreated controls. Serum biochemical analyses (glutamic oxaloacetic transaminase [GOT], glutamic pyruvic transaminase [GPT], and total protein [TP]) showed no significant toxicity; however, GOT and GPT levels were slightly decreased in the LCM‐treated group, indicating a potential anti‐inflammatory role of LCM, which warrants further investigation. This study demonstrates the effectiveness and safety of AMX and LCM in treating streptococcal infections in starry flounders and highlights the potential of the antibiotics as therapeutic agents in aquaculture. Future studies should investigate whether LCM modulates inflammatory pathways and elucidates the underlying molecular mechanisms in fish.

The body color state is an important determinant of the value of golden severum (Heros efasciatus)—a popular ornamental fish. The use of dietary supplements to improve the color development and health of this species is unexplored. Herein, the effects of marigold extract (MG) and carophyll red (CR) are examined on the growth, body color development, antioxidant properties, and innate immunity in golden severum. Fish were maintained under controlled water quality conditions (pH, temperature, and dissolved oxygen) and fed six experimental diets containing either 0% MG and CR, 1% MG, 2% MG, 5% MG, 0.5% CR, or 2% CR for five weeks. Both MG and CR significantly decreased lipid peroxide levels in hepatic tissues. In contrast, only MG enhanced the activities of reactive oxygen species (ROS)-scavenging enzymes (superoxide dismutase and catalase). Although MG and CR decreased the respiratory burst activity of splenic leukocytes, other innate immune parameters remained unchanged. Additionally, MG and CR stimulated body color development patterns in golden severum that reflect their unique coloring principles. The ROS-scavenging abilities of MG and CR appear to be related to their antioxidant activity. Hence, MG and CR at the optimal levels of 1.0% and 0.5%, respectively, can improve the body color of golden severum and protect against oxidative stress.

Enrofloxacin (ENR) is a fluoroquinolone antimicrobial used to treat bacterial diseases in aquaculture and is metabolized into ciprofloxacin (CIP). This study investigated the serum pharmacokinetics and tissue distribution of ENR and CIP in black rockfish (Sebastes schlegelii) following a single intraperitoneal administration of ENR 10 mg/kg at 13 °C and 22 °C. Absorption, distribution, and elimination of ENR were accelerated at 22 °C compared with 13 °C, with the elimination half–life decreasing from 35.75 h at 13 °C to 21.11 h at 22 °C. Kidney levels indicated higher ENR values than serum, liver, and muscle levels at both temperatures. CIP was formed only at low levels (2.13–8.05%), with greater formation at 22 °C. The temperature–elimination relationship observed after intraperitoneal administration was opposite to that reported for oral administration under the same conditions, suggesting the occurrence of flip–flop kinetics at high water temperature following oral administration. These findings indicate that both water temperature and route of administration should be considered when evaluating the pharmacokinetics of ENR in fish.